Abstract: When a mine closes, the open pit left behind often fills with water that washes metals from the pit walls. This water may be toxic and cannot be disharged without further treatment. This study evaluated the possibility of using algal blooms to remove toxic metals from pit lakes. The study had two components: laboratory tests and pit lake fertilization trials. The site for the field trial was Little Creek Pond, a small collection pond located at Vangorda Pit, near Faro, Yukon. The pond receives zinc-contaminated leachate from a waste rock dump and seepage from Vangorda Pit.Leachate is water that has percolated through soil or rock containing soluble substances such as zinc and that contains certain amounts of these substances in solution.
The laboratory study, using Little Creek Pond water, demonstrated that zinc could be removed by promoting an algal bloom. Its concentrations were decreased dramatically during the 56-day study (from starting concentrations of 55 mg/L down to 3 mg/L). An interesting finding was that zinc was removed if pond sediments were present, but not if they were absent. The high initial zinc concentrations may have been too toxic to allow the development of a strong algal bloom in the absence of sediments. This idea is supported by other recent studies, which reported that zinc could be removed from the water column at lower initial concentrations.
During the short field season at Little Creek Pond, good algal growth was promoted on pond sediments by addition of fertilizer. This is remarkable, considering that pond water contained over 200 mg/L, a highly toxic level. Nevertheless, zinc removal was not shown in the field trial. This may be partially explained by the short time abailable to produce the algal blooms and the fact that metal laden waters were continuously seeping into Little Creek Pond during the study. Our attempt to determine if zinc removal still occurred was frustrated by our inability to calculate an accurate water balance. This was due to a lack of reliable data on seepage flows and zinc concentrations, on water volumes in the pond, and on other possible inputs into the pond from groundwater.
A single measurement of zinc content in harvested algae (over 12g per kg wet weight) indicates that zinc was successfully absorbed by algae growing on sediments. However, this provides insufficient information to conclude that significant zinc removal occurred.
Despite these qualified results, aquatic phytoremediation appears to be a promising, low cost alternative to conventional lime treatment which has been used at mine sites in the past. We recommend that further studies be undertaken in pit lakes that contain lower zinc concentrations, such as the Grum pit, near Faro, Yukon.

Authors: Microbial Technologies, Inc. and Laberge Environmental Services, Inc.

Map Scale: 1 : 0

NTS Mapsheet(s): 105M

Citation: Microbial Technologies, Inc. and Laberge Environmental Services, Inc., 2003. Detoxifying Pit Lakes by Controlled Algal Blooms: Laboratory Study and Pilot Field Trial at little Creek Pond at Vangoda Pit near Faro, Yukon, Canada. Mining & Petroleum Environment Research Group, 2003-1.

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